The present invention relates to a hydraulic brake system with slip control.
In prior-art brake systems of this type, for example, DE-OS 33 23 402, the braking pressure is influenced in response to the rotational behavior of the wheels by means of electromagnetically operable inlet and outlet valves in order to prevent locking of the wheels due to excessive brake application, on the one hand, and to conform the brake force distribution to the front and the rear wheels to the prevailing braking situation, on the other hand. The provision of controlling the brake force distribution permits taking into consideration the influence of the effects of varying load conditions of a vehicle under static or dynamic axle-load shift. To preserve driving stability, it is necessary that the rear wheels will not lock before the front wheels.
According to the above-mentioned German patent application, inlet valves, which are multiple-way valves closed in their inactive position, are inserted into the main pressure lines leading to the rear wheels to control the brake force distribution. In contrast thereto, the inlet valves leading to the front wheels are open in their inactive position permitting the passage of pressure medium. Due to clockwise actuation of the valves, an amount of brake slip results at the rear wheels which is always smaller than that at the front wheels. This way, the rear wheels contribute a great deal to deceleration, while, simultaneously, over-braking of the rear wheels is prevented.
As disclosed in the above cited publication, the inlet and the outlet valves, the wheel sensors, the analyzing electronics and the auxiliary-pressure supply system are used for the control of the brake force distribution as well as for the anti-lock control, but the disadvantage is that no braking effect will result at the rear axle upon the occurrence of a malfunction in the electric or electronic unit, which activates the inlet valves of the rear-axle brake.
Further, DE-OS 34 40 541 discloses a brake system which is equipped with wheel valves exclusively for the electronic control of the brake force distribution. Therefore, only the brake lines leading to the rear-wheel brakes contain wheel valves, i.e. solenoid valves closed in their inactive position. Connected in parallel to the wheel valves is a brake force regulator, through which, on brake application, braking pressure propagates to the rear-wheel brakes which is conformed to the minimum pressure in response to the respective deceleration, with ideal brake force distribution. The braking pressure is raised to a higher value which is optimally adapted to the instantaneous situation by actuation of the wheel valves. This arrangement is based on the consideration that a minimum amount of braking pressure be conveyed to the rear-wheel brakes by way of the brake force regulator even upon power failure or a valve defect so that the rear axle can participate in braking the vehicle even when such malfunctions occur. However, the disadvantage of such a brake system is that the effect of the brake force distributor does not reach the optimal, theoretically possible value. On the other hand, the pressure transmitted via the parallel branch may already cause wheel lock in certain braking situations.